1. The Field of the Invention
The invention relates to an illumination system having a light-emitting panel and a light mixing chamber for illuminating a display device.
2. Description of the Related Art
Such illumination systems are known per se and are alternatively referred to as edge-lighting systems. They are used, inter alia, as backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as a backlight for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones.
Typically, known display devices generally include a substrate provided with a regular pattern of pixels, which are each driven by at least one electrode. In order to reproduce an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device, the display device uses a control circuit. In an LCD device, the light originating from the backlight is modulated by a switch or a modulator, while applying various types of liquid crystal effects. In addition, the display may be based on electrophoretic or electromechanical effects.
In the known illumination systems, customarily a tubular low-pressure mercury-vapor discharge lamp, for example one or more compact fluorescent lamps, is used as the light source, the light which, in operation, is emitted by the light source being coupled into the light-emitting panel which serves as an optical waveguide. This optical waveguide generally forms a comparatively thin and flat panel which is made, for example, of a synthetic resin or glass, light being transported through the optical waveguide under the influence of (total) internal reflection.
Such an illumination system may also be provided with an alternative light source in the form of a plurality of optoelectronic elements, also referred to as electro-optic elements, such as electroluminescent elements, for example light-emitting diodes (LEDs). These light sources are generally provided in the proximity of or contiguous to a light-transmitting edge area of the light-emitting panel, so that, in operation, light originating from the light source is incident on the light-transmitting edge area and spreads in the panel.
U.S. Pat. No. 5,613,751 discloses an illumination system comprising a light-emitting panel and one or more light sources, which are arranged in a so-called light transition area which is coupled to or integral with the light-emitting panel, with the distribution of the light coupled out of the light-emitting panel to illuminate the display device being brought about by providing a suitable pattern of deformities in the light-emission window of the light-emitting panel.
An illumination system of the above-mentioned type has the disadvantage that the light distribution in the light-emitting panel, particularly in the proximity of the light source, is insufficiently uniform. As a result, the illumination uniformity of the display device is insufficient.
It is an object of the invention to completely or partly overcome the above-mentioned drawback. The invention more particularly aims at providing an illumination system, wherein the uniformity of the light distribution of the illumination system and hence the uniformity with which the display device is illuminated are improved.
In accordance with the invention, this object is achieved in that
the light source comprises a plurality of clusters of light-emitting diodes having different light-emission wavelengths, said clusters being arranged at a pitch P with respect to each other, and
the ratio between a height H of the light mixing chamber and the pitch P of the clusters of the light-emitting diodes meets the relation:
0.1xe2x89xa6H/Pxe2x89xa610,
the height H of the light mixing chamber being measured at right angles to the light-coupling edge area.
As the light-emitting diodes are clustered, mixing of the light-emitted by the light-mitting diodes (LEDs) is enhanced. An example of such a cluster is a combination of one lue LED, one green LED and one red LED. An alternative embodiment of such a cluster comprises one blue LED and one red LED in combination with two green LEDs. In addition to the above-mentioned color combinations, a cluster may comprise an amber LED.
The application of a light mixing chamber in accordance with the measure of the invention has the advantage that the light originating from the LEDs of different colors mixes well. A geometric ratio of the height of the light mixing chamber to the pitch of the clusters of LEDs in the range between said limits causes light originating from the LEDs, which is coupled into the light-emitting panel via the light-coupling edge area, to have been sufficiently mixed in the light mixing chamber when it is coupled out of the light emission window to uniformly illuminate the (image) display device.
The light mixing chamber in accordance with the invention is of simple construction and inexpensive to manufacture. This can be attributed, among other things, to the fact that it is not necessary to provide additional optical structures in the light mixing chamber to mix light originating from the LEDs of different colors. Such optical structures comprise optical elements which may, or may not, be provided with intricately curved surfaces which may, or may not, be provided with reflective coatings. Such optical elements are used to diffuse the light, emitted by the LEDs in the direction of the light-coupling edge area of the light-emitting panel, in directions along the longitudinal direction of the light mixing chamber, thereby enhancing the mixing of the light.
The lower limit of the ratio of the height of the light mixing chamber to the pitch of the clusters (H/P=0.1) is motivated by the fact that it is desirable that light beams originating from the LEDs of different colors in adjoining clusters at least touch each other at the location of the light-coupling edge area. The upper limit of the above-mentioned height/pitch ratio (H/P=10) is motivated by the fact that, in the absence of such an upper limit, (the height of) the light mixing chamber would become impracticably large and hence the edge of the (image) display device impracticably wide.
By virtue of the measure in accordance with the invention, the uniformity of the distribution of the light emitted by the illumination system is improved. As a result, a more uniform illumination of the (image) display device is obtained.
Preferably, the ratio between the height (H) of the light mixing chamber and the pitch (P) of the clusters meets the relation:
0.2xe2x89xa6H/Pxe2x89xa62.
The lower limit of the ratio between the height of the light mixing chamber and the pitch of the clusters (H/P=0.2) is motivated by the fact that it is desirable that light beams originating from LEDs of different colors in adjoining clusters demonstrate at least a 50% overlap at the location of the light-coupling edge area. As a result, the uniformity of the distribution of the light emitted by the illumination system is further improved. The upper limit of said height/pitch ratio (H/P=2) is dictated by the fact that, in the absence of such an upper limit, (the height of) the light mixing chamber becomes larger than the height customarily applied in (image) display devices.
The positioning of the LEDs with respect to each other in a cluster influences the way in which light issues from the light-emitting panels. A favorable embodiment of the illumination system in accordance with the invention is characterized in that the ratio between the height H of the light mixing chamber and the pitch P of the clusters of the light-emitting diodes meets the relation:             1              2        ⁢                  xe2x80x83                ⁢        tan        ⁢                  xe2x80x83                ⁢                  β          max                      ≤          H      P        ≤          2              tan        ⁢                  xe2x80x83                ⁢                  β          max                      ,
where xcex2 is the light emission angle of the light-emitting diode in a plane parallel to the longitudinal direction of the light mixing chamber and measured with respect to a longitudinal axis of the light-emitting diode, and
where xcex2max is the maximum value of xcex2 at which the light-emitting diode emits, in operation, 90% of its energy for values of xcex2 at which |xcex2|xe2x89xa6xcex2max.
The angle xcex2 represents the aperture angle of the light which, in operation, is emitted by the LED in the longitudinal direction of the light mixing chamber. Within the limits for which it applies that xe2x88x92xcex2maxxe2x89xa6xcex2xe2x89xa6xcex2max the LED emits, in operation, 90% of its light. As a result of said choice of the orientation of the angle xcex2, the above-mentioned relation is given a measure regarding the degree of mixing of the various colors of light originating from the LEDs in adjoining clusters. If a certain cluster consists, by way of example, of a succession of one blue LED, one green LED and one red LED, and adjoining clusters have the same sequence of LEDs, it is desirable for the light originating from a LED of a certain color of said cluster to have a light distribution, upon reaching the light-coupling edge area, such that at least a part of the light beam of said LED coincides with the light originating from the light beam of the LED of the same color in the adjoining cluster.
The lower limit 2tan(xcex2max)=P/H in the above-mentioned formula is motivated by the fact that the light beams originating from LEDs of the same color in adjoining clusters at least contact each other at the location of the light-coupling edge area of the light-emitting panel. If said light beams do not contact each other, then there is a region on the light-coupling edge area where no direct light is incident of said color originating from LEDs of said color in said adjoining clusters. The upper limit tan(xcex2max)=2P/H in the above-mentioned formula is motivated by the fact that it is sufficient if the light beam originating from a LED of a certain color in a cluster has a width such that, at the location of the light-coupling edge area, the light beam still emits light at a distance of twice the pitch of the clusters (2xc3x97P). In other words, the upper limit is attained at a light distribution of the LEDs at which the light emitted by an LED of a specific color in a cluster is such that the light beam of said LED contacts, at the location of the light-coupling edge area, a perpendicular line dropped from an LED of the same color in a cluster adjoining the adjoining cluster of LEDs. By virtue of said measure, the uniformity of the distribution of the light emitted by the illumination system is further improved.
An alternative, favorable embodiment of the illumination system in accordance with the invention is characterized in that the ratio between the height H and a width D of the light mixing chamber meets the relation:             1              2        ⁢                  xe2x80x83                ⁢        tan        ⁢                  xe2x80x83                ⁢                  α          max                      ≤          H      D        ≤          3              tan        ⁢                  xe2x80x83                ⁢                  α          max                      ,
where D is the width of the light mixing chamber measured at right angles to the longitudinal direction of the light mixing chamber,
where xcex1 is the light emission angle of the light-emitting diode in a plane transverse to the longitudinal direction of the light mixing chamber and measured with respect to a longitudinal axis of the light-emitting diode, and
where xcex1max is the maximum value of xcex1 at which the light-emitting diode emits, in operation, 90% of its energy for values of xcex1 at which |xcex1|xe2x89xa6xcex1max.
The angle xcex1 represents the aperture angle of the light which, in operation, is emitted by the LED in a direction transverse to the longitudinal direction of the light mixing chamber. Within the limits for which it applies that xe2x88x92xcex1maxxe2x89xa6xcex1xe2x89xa6xcex1max, the LED emits, in operation, 90% of its light. As a result of said choice of the orientation of the angle xcex1a, the above-mentioned relation H/D is given a measure regarding the number of times that a light beam originating from the LED and emitted at a given angle xcex1 collides with a wall of the light mixing chamber (and is reflected) before said light beam reaches the light-coupling edge area.
The lower limit 2tan(xcex1max)=D/H in the above-mentioned formula is motivated by the fact that, at the location of the light-coupling edge area, the light beams originating from the LEDs comprise exactly the width of the light-coupling edge area. If xcex1max is smaller than the above-mentioned lower limit, then the light-coupling edge area is not entirely illuminated. If xcex1max is equal to the lower limit mentioned above, then no reflection of light at the walls of the light mixing chamber takes place in the plane formed by the angle xcex1 (=the plane at the location of the LED which extends transversely to the longitudinal direction of the light mixing chamber). If xcex1max is larger than said lower limit, reflections of light occur at the walls of the light mixing chamber. Although it has been found in experiments that these reflections, in principle, do not contribute to the mixing of the light in said plane, said reflections may help to mix the light in the plane formed by the angle xcex1 At each reflection, a (small) part of the light is lost. Therefore, the upper limit tan(xcex1max)=3D/H in the above-mentioned formula is motivated by the fact that it is amply sufficient if a part of the light beams emitted by the LEDs reflect maximally three times (3) at the wall of the light mixing chamber before said light beams reach the light-coupling edge area. By virtue of said measure, the uniformity of the distribution of the light emitted by the illumination system is further improved.
A synergetic effect is achieved by using a light mixing chamber having a height H and a width D, which, given the positioning and light distribution of the LEDs (xcex1max and xcex1max), meet the two above-mentioned relations for H/P and H/D.
Preferably, the LEDs have a wide light distribution, particularly in the longitudinal direction of the light mixing chamber. A wide light distribution enhances the mixing of the light in (the longitudinal direction of) the light mixing chamber. It is particularly favorable if the value of the angle xcex2max lies in the range from 60xc2x0xe2x89xa6xcex2maxxe2x89xa680xc2x0. The higher the value of xcex2max, the more light is emitted by the LEDs in the longitudinal direction of the light mixing chamber.
If the light-emitting diodes are provided in a wall of the light mixing chamber which is remote from the light-coupling edge area and extends parallel to the light-coupling edge area, it is particularly favorable if the value of the angle xcex1max lies in the range from 20xe2x89xa6xcex1maxxe2x89xa650xc2x0. If the light-emitting diodes are alternatively provided in a wall of the light mixing chamber transverse to the light-coupling edge areas, it is particularly favorable if the value of the angle xcex1max lies in the range from 60xc2x0xe2x89xa6xcex1maxxe2x89xa680xc2x0.
In order to further enhance the mixing of light originating from the light mixing chamber, means for further mixing the light emitted by the plurality of LEDs of different colors may be situated between the light mixing chamber and the light-coupling edge area. For this purpose, a preferred embodiment of the illumination system is characterized in that a diffuser is present between the light mixing chamber and the light-coupling edge area. An additional advantage resides in that such a diffuser enables effects of non-identical light beam distributions of LEDs of different colors to be further averaged out.
A favorable embodiment of the illumination system is characterized in that the panel comprises two light-coupling edge areas, which are situated at opposite sides of the panel. In an alternative embodiment of the illumination system, the light-coupling edge area is situated at the side of the light-emitting panel that is opposite the light emission window (a so-called direct backlight).
The light-emitting diodes having different light-emission wavelengths are preferably clustered together in clusters comprising:
one blue, one green and one red light-emitting diode,
one blue, two green and one red light-emitting diodes,
one blue, one green, one amber and one red light-emitting diode, and
one blue, two green, one amber and one red light-emitting diodes.
Preferably, each of the light-emitting diodes comprises a luminous flux of at least 5 lm. LEDs having such a high output are alternatively referred to as LED power packages. The application of these high-efficiency, high-output LEDs has the specific advantage that the number of LEDs at a desired, comparatively high light output can be comparatively small. This has a favorable effect on the compactness and the efficiency of the illumination system to be manufactured.
Further advantages of the use of LEDs are: a comparatively very long service life, comparatively low energy costs and low maintenance costs of an illumination system comprising LEDs. In addition, the use of LEDs has the advantage that dynamic illumination possibilities are obtained.
In a further preferred embodiment, the illumination system comprises control electronics for changing the luminous flux of the light-emitting diodes. Suitable control electronics enables the desired illumination effects to be achieved and the uniformity of the coupled-out light to be improved. In addition, a suitable combination of LEDs enables white light to be obtained, and a desired color temperature can be obtained by control electronics.
Preferably, the LEDs are driven per string of serially coupled LEDs, whereby a string comprises LEDs of substantially the same light-emission wavelength. For example, all blue LEDs provided in a light mixing chamber in accordance with the invention jointly form one string. The advantage of driving the LEDs per string is that it results in a suitable drive voltage at a substantially equal luminous flux per string of LEDs.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.